Image coding device, image decoding device, image coding method, and image decoding method

ABSTRACT

When removing a block distortion occurring in a local decoded image, a loop filtering part 11 of an image coding device carries out a filtering process on each of signal components (a luminance signal component and color difference signal components) after setting the intensity of a filter for removing the block distortion for each of the signal components according to a coding mode (an intra coding mode or an inter coding mode) selected by a coding controlling part 1.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of co-pending application Ser. No.17/083,441, filed Oct. 29, 2020, which is a Divisional of applicationSer. No. 16/399,178, filed on Apr. 30, 2019 (now U.S. Pat. No.10,863,180 issued Dec. 8, 2020), which is a Divisional of applicationSer. No. 15/868,253, filed on Jan. 11, 2018 (now U.S. Pat. No.10,334,251 issued on Jun. 25, 2019), which is a Divisional ofapplication Ser. No. 15/479,584, filed on Apr. 5, 2017 (now U.S. Pat.No. 9,906,795 issued on Feb. 27, 2018), which is a Divisional ofapplication Ser. No. 14/707,940, filed May 8, 2015 (now U.S. Pat. No.9,654,773 issued on May 16, 2017), which is a Divisional of applicationSer. No. 14/110,366, filed on Oct. 7, 2013 (now U.S. Pat. No. 9,210,426issued Dec. 8, 2015), which was filed as PCT International ApplicationNo. PCT/JP2012/003680 on Jun. 5, 2012, which claims the benefit under 35U.S.C. § 119(a) to Patent Application No. 2011-145612, filed in Japan onJun. 30, 2011, all of which are hereby expressly incorporated byreference into the present application.

FIELD OF THE INVENTION

The present invention relates to an image coding device for and an imagecoding method of compression-coding an image and transmitting thisimage, and an image decoding device for and an image decoding method ofdecoding coded data transmitted from an image coding device to acquirean image.

BACKGROUND OF THE INVENTION

Conventionally, in accordance with an international standard videocoding method, such as MPEG or ITU-T H.26x, an inputted video frame isdivided into macroblocks each of which consists of a 16×16-pixel block,a motion-compensated prediction is carried out on each macroblock, and,after that, orthogonal transformation and quantization are carried outon a prediction error signal in units of a block in order to compressthe information about the inputted video frame. A problem with such aninternational standard video coding method is, however, that as thecompression ratio becomes high, the compression efficiency is reduceddue to reduction in the quality of a prediction reference image which isused when carrying out the motion-compensated prediction. To solve thisproblem, in accordance with an MPEG-4 AVC/H.264 coding method (refer tononpatent reference 1), a block distortion occurring in a predictionreference image through quantization of orthogonal transformationcoefficients is removed by carrying out an in-loop blocking filteringprocess.

FIG. 16 is a block diagram showing an image coding device disclosed bynonpatent reference 1. In this image coding device, when receiving animage signal which is a target to be coded, a block dividing part 101divides the image signal into image signals about macroblocks, andoutputs the image signal about each of the macroblocks as a dividedimage signal to a predicting part 102. When receiving the divided imagesignal from the block dividing part 101, the predicting part 102 carriesout an intra-frame or inter-frame prediction on an image signal of eachcolor component in each of the macroblocks to calculate a predictiondifference signal.

Particularly, when carrying out an inter-frame motion-compensatedprediction on the image signal of each color component, the predictingpart searches through each macroblock itself or each of subblocks intowhich each macroblock is divided more finely for a motion vector. Thepredicting part then carries out a motion-compensated prediction on areference image signal stored in a memory 107 by using the motion vectorto generate a motion-compensated prediction image, and calculates aprediction difference signal by acquiring the difference between aprediction signal showing the motion-compensated prediction image andthe divided image signal. The predicting part 102 also outputsparameters for prediction signal generation which the predicting partdetermines when acquiring the prediction signal to a variable lengthcoding part 108. For example, the parameters for prediction signalgeneration includes information such as a motion vector showing aninter-frame motion amount.

When receiving the prediction difference signal from the predicting part102, a compressing part 103 removes a signal correlation from theprediction difference signal by carrying out a DCT (discrete cosinetransform) process on the prediction difference signal, and, after that,acquires compressed data by quantizing the prediction difference signalfrom which the signal correlation is removed. When receiving thecompressed data from the compressing part 103, a local decoding part 104inverse-quantizes the compressed data and carries out an inverse DCTprocess on the compressed data inverse-quantized thereby, and calculatesa prediction difference signal corresponding to the predictiondifference signal outputted from the predicting part 102.

When receiving the prediction difference signal from the local decodingpart 104, an adder 105 adds the prediction difference signal and theprediction signal outputted from the predicting part 102 to generate alocal decoded image. A loop filter 106 removes a block distortionpiggybacked onto a local decoded image signal showing the local decodedimage generated by the adder 105, and stores the local decoded imagesignal from which the distortion is removed as a reference image signalin the memory 107.

When receiving the compressed data from the compressing part 103, thevariable length coding part 108 entropy-encodes the compressed data andoutputs a bitstream which is the coded result. When outputting thebitstream, the variable length coding part 108 multiplexes theparameters for prediction signal generation outputted from thepredicting part 102 into the bitstream and outputs this bitstream.

According to a method disclosed by nonpatent reference 1, the loopfilter 106 determines a smoothing intensity (filter intensity) for eachneighboring pixel at a block boundary of DCT on the basis of informationincluding the roughness of the quantization, the coding mode, the degreeof variation in the motion vector, etc., and carries out a filteringprocess on the local decoded image so as to provide a reduction in adistortion (block distortion) occurring at a block boundary. As aresult, the quality of the reference image signal can be improved, andthe efficiency of the motion-compensated prediction in subsequent codingcan be improved.

RELATED ART DOCUMENT Nonpatent Reference

[Nonpatent Reference 1]

-   Nonpatent reference: MPEG-4 AVC (ISO/IEC 14496-10)/H.ITU-T 264    standards

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Because the conventional image coding device is constructed as above,when the loop filter 106 removes a block distortion occurring in ablock, the intensity of the filter for the luminance signal component ofthe block is determined on the basis of the coding mode, etc., while theintensity of the filter for the luminance signal component is also usedas the intensity of the filter for each of the color difference signalcomponents of the block. Therefore, a problem is that the intensity ofthe filter for each of the color difference signal components is notnecessarily appropriate and the improvement of the image quality isrestricted.

The present invention is made in order to solve the above-mentionedproblem, and it is therefore an object of the present invention toprovide an image coding device and an image coding method capable ofimproving the accuracy of block distortion removal, thereby improvingthe quality of a coded image. It is another object of the presentinvention to provide an image decoding device and an image decodingmethod capable of improving the accuracy of block distortion removal,thereby improving the quality of a decoded image.

Means for Solving the Problem

In accordance with the present invention, there is provided an imagecoding device including: a block dividing unit for dividing an inputtedimage into blocks each of which is a unit for coding process; a codingmode determining unit for determining a coding mode for each of theblocks into which the inputted image is divided by the block dividingunit; a prediction image generating unit for carrying out a predictionprocess on each of the blocks into which the inputted image is dividedby the block dividing unit to generate a prediction image whilereferring to a local decoded image of an already-coded block accordingto the coding mode determined by the coding mode determining unit; adifference image generating unit for generating a difference imagebetween each of the blocks into which the inputted image is divided bythe block dividing unit, and the prediction image generated by theprediction image generating unit; an image compression unit forcompressing the difference image generated by the difference imagegenerating unit, and outputting compressed data of the difference image;a local decoded image generating unit for decompressing the differenceimage compressed by the image compression unit, and adding thedifference image decompressed thereby and the prediction image generatedby the prediction image generating unit to generate a local decodedimage; a distortion removing unit for carrying out a filtering processon the local decoded image generated by the local decoded imagegenerating unit to remove a block distortion occurring in the localdecoded image; and a coding unit for coding the compressed dataoutputted from the image compression unit and the coding mode determinedby the coding mode determining unit to generate a bitstream into whichcoded data of the compressed data and coded data of the coding mode aremultiplexed, in which when removing a block distortion occurring in thelocal decoded image, the distortion removing unit sets an intensity of afilter for removing the block distortion for each signal componentaccording to the coding mode determined by the coding mode determiningunit.

Advantages of the Invention

Because the image coding device in accordance with the present inventionis constructed in such a way that the image coding device includes: theblock dividing unit for dividing an inputted image into blocks each ofwhich is a unit for coding process; the coding mode determining unit fordetermining a coding mode for each of the blocks into which the inputtedimage is divided by the block dividing unit; the prediction imagegenerating unit for carrying out a prediction process on each of theblocks into which the inputted image is divided by the block dividingunit to generate a prediction image while referring to a local decodedimage of an already-coded block according to the coding mode determinedby the coding mode determining unit; the difference image generatingunit for generating a difference image between each of the blocks intowhich the inputted image is divided by the block dividing unit, and theprediction image generated by the prediction image generating unit; theimage compression unit for compressing the difference image generated bythe difference image generating unit, and outputting compressed data ofthe difference image; the local decoded image generating unit fordecompressing the difference image compressed by the image compressionunit, and adding the difference image decompressed thereby and theprediction image generated by the prediction image generating unit togenerate a local decoded image; the distortion removing unit forcarrying out a filtering process on the local decoded image generated bythe local decoded image generating unit to remove a block distortionoccurring in the local decoded image; and the coding unit for coding thecompressed data outputted from the image compression unit and the codingmode determined by the coding mode determining unit to generate abitstream into which coded data of the compressed data and coded data ofthe coding mode are multiplexed, in which when removing a blockdistortion occurring in the local decoded image, the distortion removingunit sets an intensity of a filter for removing the block distortion foreach signal component according to the coding mode determined by thecoding mode determining unit, there is provided an advantage of beingable to improve the accuracy of removal of a block distortion, therebyimproving the quality of the decoded image.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram showing an image coding device in accordancewith Embodiment 1 of the present invention;

FIG. 2 is a flow chart showing processing carried out by the imagecoding device in accordance with Embodiment 1 of the present invention;

FIG. 3 is a block diagram showing an image decoding device in accordancewith Embodiment 1 of the present invention;

FIG. 4 is a flow chart showing processing carried out by the imagedecoding device in accordance with Embodiment 1 of the presentinvention;

FIG. 5 is an explanatory drawing showing a state in which each codingblock having a maximum size is hierarchically divided into a pluralityof coding blocks;

FIG. 6(a) is an explanatory drawing showing a distribution of partitionsinto which a coding block is divided, and FIG. 6(b) is an explanatorydrawing showing a state in which a coding mode m(B^(n)) is assigned toeach of the partitions after a hierarchical layer division is performedby using a quadtree graph;

FIG. 7 is an explanatory drawing showing the positions of pixels in acoding block to each of which a filter is applied;

FIG. 8 is a flow chart showing processing carried out by a loopfiltering part 11;

FIG. 9 is a flow chart showing a method of determining a filterintensity;

FIG. 10 is an explanatory drawing showing a relationship between theposition of an edge and the positions of pixels;

FIG. 11 is an explanatory drawing showing a unit in which a filteringprocess is carried out on a vertical edge;

FIG. 12 is an explanatory drawing showing a unit in which a filteringprocess is carried out on a horizontal edge;

FIG. 13 is an explanatory drawing showing a correspondence between Q (qPvalue of luminance) and parameters β and Tc;

FIG. 14 is an explanatory drawing showing a bitstream generated by avariable length coding part 13;

FIG. 15 is an explanatory drawing showing an example in which the sizeof a coding block B^(n) is L^(n)=kM^(n); and

FIG. 16 is a block diagram showing an image coding device disclosed innonpatent reference 1.

EMBODIMENTS OF THE INVENTION

Hereafter, in order to explain this invention in greater detail, thepreferred embodiments of the present invention will be described withreference to the accompanying drawings.

Embodiment 1

In this Embodiment 1, an image coding device that receives each frameimage of a video, carries out variable length coding on the frame imageafter carrying out a compression process with an orthogonaltransformation and quantization on a prediction difference signal whichthe image coding device acquires by carrying out a motion-compensatedprediction between adjacent frames to generate a bitstream, and an imagedecoding device that decodes the bitstream outputted from the imagecoding device will be explained.

The image coding device in accordance with this Embodiment 1 ischaracterized in that the image coding device adapts itself to a localchange of a video signal in space and time directions to divide thevideo signal into regions of various sizes, and carries out intra-frameand inter-frame adaptive coding. In general, a video signal has acharacteristic of its complexity varying locally in space and time.There can be a case in which a pattern having a uniform signalcharacteristic in a relatively large image area, such as a sky image ora wall image, or a pattern having a complicated texture pattern in asmall image area, such as a person image or a picture including a finetexture, also coexists on a certain video frame from the viewpoint ofspace. Also from the viewpoint of time, a relatively large image area,such as a sky image or a wall image, has a small local change in a timedirection in its pattern, while an image of a moving person or objecthas a larger temporal change because its outline has a movement of arigid body and a movement of a non-rigid body with respect to time.

Although in the coding process a process of generating a predictiondifference signal having small signal power and small entropy by using atemporal and spatial prediction, thereby reducing the whole code amount,is carried out, the code amount of a parameter used for the predictioncan be reduced as long as the parameter can be applied uniformly to aslarge an image signal region as possible. On the other hand, because theamount of errors occurring in the prediction increases when the sameprediction parameter is applied to an image signal pattern having alarge change in time and space, the code amount of the predictiondifference signal cannot be reduced. Therefore, it is desirable toreduce the size of a region which is subjected to the prediction processwhen performing the prediction process on an image signal pattern havinga large change in time and space, thereby reducing the electric powerand entropy of the prediction difference signal even though the datavolume of the parameter which is used for the prediction is increased.In order to carry out coding which is adapted for such the typicalcharacteristics of a video signal, the image coding device in accordancewith this Embodiment 1 hierarchically divides each region having apredetermined maximum block size of the video signal into blocks, andcarries out the prediction process and the coding process of coding theprediction difference on each of the blocks into which each region isdivided.

A video signal which is to be processed by the image coding device inaccordance with this Embodiment 1 can be an arbitrary video signal inwhich each video frame consists of a series of digital samples (pixels)in two dimensions, horizontal and vertical, such as a YUV signal whichconsists of a luminance signal and two color difference signals, a colorvideo image signal in arbitrary color space, such as an RGB signal,outputted from a digital image capturing sensor, a monochrome imagesignal, or an infrared image signal. The gradation of each pixel can bean 8-bit, 10-bit, or 12-bit one. In the following explanation, theinputted video signal is a YUV signal unless otherwise specified. It isfurther assumed that the two color difference components U and V aresignals having a 4:2:0 format which are subsampled with respect to theluminance component Y. A data unit to be processed which corresponds toeach frame of the video signal is referred to as a “picture.” In thisEmbodiment 1, a “picture” is explained as a video frame signal on whichprogressive scanning has been carried out. When the video signal is aninterlaced signal, a “picture” can be alternatively a field image signalwhich is a unit which constructs a video frame. Further, in thesubsequent explanation, a group of spatially continuous coding blocksmay be referred to as a “slice.”

FIG. 1 is a block diagram showing the image coding device in accordancewith Embodiment 1 of the present invention. Referring to FIG. 1 , acoding controlling part 1 carries out a process of determining a maximumsize of each of coding blocks which is a unit to be processed at a timewhen a motion-compensated prediction process (inter-frame predictionprocess) or an intra prediction process (intra-frame prediction process)is carried out, and also determining an upper limit on the number ofhierarchical layers in a hierarchy in which each of the coding blockshaving the maximum size is hierarchically divided into blocks. Thecoding controlling part 1 also carries out a process of selecting acoding mode suitable for each of the coding blocks into which eachcoding block having the maximum size is divided hierarchically from oneor more available coding modes (one or more intra coding modes and oneor more inter coding modes). The coding controlling part 1 constructs acoding mode determining unit.

A block dividing part 2 carries out a process of, when receiving a videosignal showing an inputted image, dividing the inputted image shown bythe video signal into coding blocks each having the maximum sizedetermined by the coding controlling part 1, and also dividing each ofthe coding blocks into blocks hierarchically until the number ofhierarchical layers reaches the upper limit on the number ofhierarchical layers which is determined by the coding controlling part1. The block dividing part 2 constructs a block dividing unit.

A selection switch 3 carries out a process of, when the coding modeselected by the coding controlling part 1 for the coding block, which isgenerated through the division by the block dividing part 2, is an intracoding mode, outputting the coding block to an intra prediction part 4,and, when the coding mode selected by the coding controlling part 1 forthe coding block, which is generated through the division by the blockdividing part 2, is an inter coding mode, outputting the coding block toa motion-compensated prediction part 5. The intra prediction part 4carries out a process of, when receiving the coding block, which isgenerated through the division by the block dividing part 2, from theselection switch 3, carrying out an intra prediction process on thecoding block to generate a prediction image by using an intra predictionparameter outputted from the coding controlling part 1 while referringto the local decoded image (reference image) of an already-coded blockstored in a memory 10 for intra prediction.

The motion-compensated prediction part 5 carries out a process of, whenreceiving the coding block, which is generated through the division bythe block dividing part 2, from the selection switch 3, making a motionsearch by comparing the coding block with the local decoded image(reference image) of an already-coded block stored in amotion-compensated prediction frame memory 12 to calculate a motionvector, and carries out an inter prediction process (motion-compensatedprediction process) on the coding block by using both the motion vectorand inter prediction parameters outputted from the coding controllingpart 1 to generate a prediction image. A prediction image generatingunit is comprised of the selection switch 3, the intra prediction part4, and the motion-compensated prediction part 5.

A subtracting part 6 carries out a process of subtracting the predictionimage generated by the intra prediction part 4 or the motion-compensatedprediction part 5 from the coding block, which is generated through thedivision by the block dividing part 2, to generate a difference image(=the coding block−the prediction image). The subtracting part 6constructs a difference image generating unit. Atransformation/quantization part 7 carries out a process of performingan orthogonal transformation process (e.g., a DCT (discrete cosinetransform) or an orthogonal transformation process, such as a KLtransform, in which bases are designed for a specific learning sequencein advance) on the difference image generated by the subtracting part 6in units of a block having a transformation block size included inprediction difference coding parameters outputted from the codingcontrolling part 1, and also quantizing the transform coefficients ofthe difference image by using a quantization parameter included in theprediction difference coding parameters to output the transformcoefficients quantized thereby as compressed data of the differenceimage. The transformation/quantization part 7 constructs an imagecompression unit.

An inverse quantization/inverse transformation part 8 carries out aprocess of inverse-quantizing the compressed data outputted from thetransformation/quantization part 7 by using the quantization parameterincluded in the prediction difference coding parameter outputted fromthe coding controlling part 1, and performing an inverse transformationprocess (e.g., an inverse DCT (inverse discrete cosine transform) or aninverse transformation process such as an inverse KL transform) on thecompressed data inverse-quantized thereby in units of a block having thetransformation block size included in the prediction difference codingparameters to output the compressed data on which the inversequantization/inverse transformation part has carried out the inversetransformation process as a local decoded prediction difference signal(data showing the difference image decompressed). An adding part 9carries out a process of adding the local decoded prediction differencesignal outputted from the inverse quantization/inverse transformationpart 8 and the prediction signal showing the prediction image generatedby the intra prediction part 4 or the motion-compensated prediction part5 to generate a local decoded image signal showing a local decodedimage. A local decoded image is comprised of the inversequantization/inverse transformation part 8 and the adding part 9.

The memory 10 for intra prediction is a recording medium, such as a RAM,for storing the local decoded image shown by the local decoded imagesignal generated by the adding part 9 as an image which the intraprediction part 4 will use when performing the intra prediction processthe next time.

A loop filtering part 11 carries out a process of performing a filteringprocess (loop filtering process) on the local decoded image signalgenerated by the adding part 9 to remove a distortion (block distortion)occurring at a block boundary. When removing a block distortion of thelocal decoded image, the loop filtering part 11 sets the intensity of afilter for removing the block distortion for each of the signalcomponents (the luminance signal component and the color differencesignal components) according to the coding mode (an intra coding mode oran inter coding mode) selected by the coding controlling part 1. Theloop filtering part 11 constructs a distortion removing unit.

The motion-compensated prediction frame memory 12 is a recording medium,such as a RAM, for storing the local decoded image on which the loopfiltering part 11 has carried out the filtering process as a referenceimage which the motion-compensated prediction part 5 will use whenperforming the motion-compensated prediction process the next time.

A variable length coding part 13 carries out a process ofvariable-length-coding the compressed data outputted from thetransformation/quantization part 7, the coding mode and the predictiondifference coding parameters which are outputted from the codingcontrolling part 1, and the intra prediction parameter outputted fromthe intra prediction part 4 or the inter prediction parameters(including the motion vector) outputted from the motion-compensatedprediction part 5 to generate a bitstream into which coded data of thecompressed data, coded data of the coding mode, coded data of theprediction difference coding parameters, and coded data of the intraprediction parameter or the inter prediction parameters are multiplexed.The variable length coding part 13 constructs a coding unit.

In the example of FIG. 1 , the coding controlling part 1, the blockdividing part 2, the selection switch 3, the intra prediction part 4,the motion-compensated prediction part 5, the subtracting part 6, thetransformation/quantization part 7, the inverse quantization/inversetransformation part 8, the adding part 9, the loop filtering part 11,and the variable length coding part 13, which are the components of theimage coding device, can consist of pieces of hardware for exclusive use(e.g., semiconductor integrated circuits in each of which a CPU ismounted, one chip microcomputers, or the like), respectively. As analternative, the image coding device can consist of a computer, and aprogram in which the processes carried out by the coding controllingpart 1, the block dividing part 2, the selection switch 3, the intraprediction part 4, the motion-compensated prediction part 5, thesubtracting part 6, the transformation/quantization part 7, the inversequantization/inverse transformation part 8, the adding part 9, the loopfiltering part 11, and the variable length coding part 13 are describedcan be stored in a memory of the computer and the CPU of the computercan be made to execute the program stored in the memory. FIG. 2 is aflow chart showing the processing carried out by the image coding devicein accordance with Embodiment 1 of the present invention.

FIG. 3 is a block diagram showing the image decoding device inaccordance with Embodiment 1 of the present invention. Referring to FIG.3 , a variable length decoding part 21 carries out a process ofvariable-length-decoding the coded data multiplexed into the bitstreamto acquire the compressed data, the coding mode, the predictiondifference coding parameters, and the intra prediction parameter or theinter prediction parameters (including the motion vector), which areassociated with each of coding blocks into which each frame of the videois hierarchically divided, and outputting the compressed data and theprediction difference coding parameters to an inversequantization/inverse transformation part 25 and also outputting thecoding mode, and the intra prediction parameter or the inter predictionparameters to a selection switch 22, and further outputting the codingmode to a loop filtering part 28. The variable length decoding part 21constructs a decoding unit.

The selection switch 22 carries out a process of, when the coding modeassociated with a coding block, which is outputted from the variablelength decoding part 21, is an intra coding mode, outputting the intraprediction parameter outputted thereto from the variable length decodingpart 21 to an intra prediction part 23, and, when the coding mode is aninter coding mode, outputting the inter prediction parameters outputtedthereto from the variable length decoding part 21 to amotion-compensated prediction part 24. The intra prediction part 23carries out a process of performing an intra prediction process on thecoding block to generate a prediction image by using the intraprediction parameter outputted from the selection switch 22 whilereferring to the decoded image (reference image) of an already-decodedblock stored in a memory 27 for intra prediction.

The motion compensation part 24 carries out a process of performing aninter prediction process on the coding block to generate a predictionimage by using the motion vector included in the inter predictionparameters outputted from the selection switch 22, and the decoded image(reference image) of an already-decoded block stored in amotion-compensated prediction frame memory 29. A prediction imagegenerating unit is comprised of the selection switch 22, the intraprediction part 23, and the motion compensation part 24.

The inverse quantization/inverse transformation part 25 carries out aprocess of inverse-quantizing the compressed data which is outputtedthereto from the variable length decoding part 21 by using thequantization parameter included in the prediction difference codingparameters outputted thereto from the variable length decoding part 21,and performing an inverse transformation process (e.g., an inverse DCT(inverse discrete cosine transform) or an inverse transformation processsuch as an inverse KL transform) on the compressed datainverse-quantized thereby in units of a block having the transformationblock size included in the prediction difference coding parameters, andoutputting the compressed data on which the inverse quantization/inversetransformation part has carried out the inverse transformation processas a decoded prediction difference signal (signal showing apre-compressed difference image). The inverse quantization/inversetransformation part 25 constructs a difference image generating unit.

An adding part 26 carries out a process of adding the decoded predictiondifference signal outputted thereto from the inversequantization/inverse transformation part 25 and the prediction signalshowing the prediction image generated by the intra prediction part 23or the motion-compensated prediction part 24 to generate a decoded imagesignal showing a decoded image. The adding part 26 constructs thedecoded image generating unit. The memory 27 for intra prediction is arecording medium, such as a RAM, for storing the decoded image shown bythe decoded image signal generated by the adding part 26 as an imagewhich the intra prediction part 23 will use when performing the intraprediction process the next time.

A loop filtering part 28 carries out a process of performing a filteringprocess (loop filtering process) on the decoded image signal generatedby the adding part 26 to remove a distortion (block distortion)occurring at a block boundary. When removing a block distortion of thedecoded image, the loop filtering part 28 sets the intensity of a filterfor removing the block distortion for each of the signal components (theluminance signal component and the color difference signal components)according to the coding mode (an intra coding mode or an inter codingmode) outputted thereto from the variable length decoding part 21. Theloop filtering part 28 constructs a distortion removing unit.

The motion-compensated prediction frame memory 29 is a recording medium,such as a RAM, for storing the decoded image on which the loop filterpart 28 has carried out the filtering process as a reference image whichthe motion-compensated prediction part 24 will use when performing themotion-compensated prediction process the next time.

In the example of FIG. 3 , the variable length decoding part 21, theselection switch 22, the intra prediction part 23, themotion-compensated prediction part 24, the inverse quantization/inversetransformation part 25, the adding part 26, and the loop filtering part28, which are the components of the image decoding device, can consistof pieces of hardware for exclusive use (e.g., integrated circuits ineach of which a CPU is mounted, one chip microcomputers, or the like),respectively. As an alternative, the image decoding device can consistof a computer, and a program in which the processes carried out by thevariable length decoding part 21, the selection switch 22, the intraprediction part 23, the motion-compensated prediction part 24, theinverse quantization/inverse transformation part 25, the adding part 26,and the loop filtering part 28 are described can be stored in a memoryof the computer and the CPU of the computer can be made to execute theprogram stored in the memory. FIG. 4 is a flow chart showing theprocessing carried out by the image decoding device in accordance withEmbodiment 1 of the present invention.

Next, the operation of the image coding device and the operation of theimage decoding device will be explained. First, the processing carriedout by the image coding device shown in FIG. 1 will be explained. First,the coding controlling part 1 determines a maximum size of each ofcoding blocks which is a unit to be processed at a time when amotion-compensated prediction process (inter-frame prediction process)or an intra prediction process (intra-frame prediction process) iscarried out, and also determines an upper limit on the number ofhierarchical layers in a hierarchy in which each of the coding blockshaving the maximum size is hierarchically divided into blocks (step ST1of FIG. 2 ).

As a method of determining the maximum size of each of coding blocks,for example, there can be considered a method of determining a maximumsize for all the pictures according to the resolution of the inputtedimage. Further, there can be considered a method of quantifying avariation in the complexity of a local movement of the inputted image asa parameter and then determining a small size for a picture having alarge and vigorous movement while determining a large size for a picturehaving a small movement. As a method of determining the upper limit onthe number of hierarchical layers, for example, there can be considereda method of increasing the depth of the hierarchy, i.e., the number ofhierarchical layers to make it possible to detect a finer movement asthe inputted image has a larger and more vigorous movement, ordecreasing the depth of the hierarchy, i.e., the number of hierarchicallayers as the inputted image has a smaller movement.

The coding controlling part 1 also selects a coding mode suitable foreach of the coding blocks into which each coding block having themaximum size is divided hierarchically from one or more available codingmodes (M intra coding modes and N inter coding modes) (step ST2).Although a detailed explanation of the selection method of selecting acoding mode for use in the coding controlling part 1 will be omittedbecause the selection method is a known technique, there is a method ofcarrying out a coding process on the coding block by using an arbitraryavailable coding mode to examine the coding efficiency and select acoding mode having the highest level of coding efficiency from among aplurality of available coding modes, for example.

When receiving the video signal showing the inputted image, the blockdividing part 2 divides the inputted image shown by the video signalinto coding blocks each having the maximum size determined by the codingcontrolling part 1, and also divides each of the coding blocks intoblocks hierarchically until the number of hierarchical layers reachesthe upper limit on the number of hierarchical layers which is determinedby the coding controlling part 1 (step ST3). FIG. 5 is an explanatorydrawing showing a state in which each coding block having the maximumsize is hierarchically divided into a plurality of coding blocks. In theexample of FIG. 5 , each coding block having the maximum size is acoding block B⁰ in the 0th hierarchical layer, and its luminancecomponent has a size of (L⁰, M⁰. Further, in the example of FIG. 5 , bycarrying out the hierarchical division with this coding block B⁰ havingthe maximum size being set as a starting point until the depth of thehierarchy reaches a predetermined depth which is set separatelyaccording to a quadtree structure, coding blocks B^(n) can be acquired.

At the depth of n, each coding block B^(n) is an image region having asize of (L^(n), M^(n)). Although L^(n) can be the same as or differ fromM^(n), the case of L^(n)=M^(n) is shown in the example of FIG. 5 .Hereafter, the size of each coding block B^(n) is defined as the size of(L^(n), M^(n)) in the luminance component of the coding block B^(n).

Because the block dividing part 2 carries out a quadtree division,(L^(n+1), M^(n+1))=(L^(n)/2, M^(n)/2) is always established. In the caseof a color video image signal (4:4:4 format) in which all the colorcomponents have the same sample number, such as an RGB signal, all thecolor components have a size of (L^(n), M^(n)), while in the case ofhandling a 4:2:0 format, a corresponding color difference component hasa coding block size of (L^(n)/2, M^(n)/2). Hereafter, a coding modeselectable for each coding block B^(n) in the nth hierarchical layer isexpressed as m(B^(n)).

In the case of a color video signal which consists of a plurality ofcolor components, the coding mode m(B^(n)) can be configured in such away that an individual mode is used for each color component. Hereafter,an explanation will be made by assuming that the coding mode m(B^(n))indicates the one for the luminance component of each coding blockhaving a 4:2:0 format in a YUV signal unless otherwise specified. Thecoding mode m(B^(n)) can be one of one or more intra coding modes(generically referred to as “INTRA”) or one or more inter coding modes(generically referred to as “INTER”), and the coding controlling part 1selects, as the coding mode m(B^(n)), a coding mode with the highestdegree of coding efficiency for each coding block B^(n) from among allthe coding modes available in the picture currently being processed or asubset of these coding modes, as mentioned above.

Each coding block B^(n) is further divided into one or more predictionunits (partitions) by the block dividing part, as shown in FIG. 5 .Hereafter, each partition belonging to each coding block B^(n) isexpressed as P_(i) ^(n) (i shows a partition number in the nthhierarchical layer). How the division of each coding block B^(n) intopartitions P_(i) ^(n) belonging to the coding block B^(n) is carried outis included as information in the coding mode m(B^(n)). While theprediction process is carried out on each of all the partitions P_(i)^(n) according to the coding mode m(B^(n)), an individual predictionparameter can be selected for each partition P_(i) ^(n).

The coding controlling part 1 produces such a block division state asshown in, for example, FIG. 6 for each coding block having the maximumsize, and then determines coding blocks B^(n). Hatched portions shown inFIG. 6(a) show a distribution of partitions into which each coding blockhaving the maximum size is divided, and FIG. 6(b) shows a situation inwhich coding modes m(B^(n)) are respectively assigned to the partitionsgenerated through the hierarchical layer division by using a quadtreegraph. Each node enclosed by □ shown in FIG. 6(b) is a node (codingblock B^(n)) to which a coding mode m(B^(n)) is assigned.

When the coding controlling part 1 selects an optimal coding modem(B^(n)) for each partition P_(i) ^(n) of each coding block B^(n), andthe coding mode m(B^(n)) is an intra coding mode (step ST4), theselection switch 3 outputs the partition P_(i) ^(n) of the coding blockB^(n), which is generated through the division by the block dividingpart 2, to the intra prediction part 4. In contrast, when the codingmode m(B^(n)) is an inter coding mode (step ST4), the selection switchoutputs the partition P_(i) ^(n) of the coding block B^(n), which isgenerated through the division by the block dividing part 2, to themotion-compensated prediction part 5.

When receiving the partition P_(i) ^(n) of the coding block B^(n) fromthe selection switch 3, the intra prediction part carries out an intraprediction process on the partition P_(i) ^(n) of the coding block B^(n)by using the intra prediction parameter corresponding to the coding modem(B^(n)) selected by the coding controlling part 1 to generate an intraprediction image P_(i) ^(n) while referring to the local decoded imageof an already-coded block stored in the memory 10 for intra prediction(step ST5). The intra prediction part 4 outputs the intra predictionimage P_(i) ^(n) to the subtracting part 6 and the adding part 9 aftergenerating the intra prediction image P_(i) ^(n), while outputting theintra prediction parameter to the variable length coding part 13 toenable the image decoding device shown in FIG. 3 to generate the sameintra prediction image P_(i) ^(n). Although the intra prediction part 4carries out the intra prediction process in compliance with, forexample, the algorithm determined by the AVC/H.264 standards (ISO/IEC14496-10), the algorithm with which the intra prediction part compliesis not limited to this algorithm.

When receiving the partition P_(i) ^(n) of the coding block B^(n) fromthe selection switch 3, the motion-compensated prediction part 5 makes amotion search by comparing the partition P_(i) ^(n) of the coding blockB^(n) with the local decoded image of an already-coded block stored inthe motion-compensated prediction frame memory 12 to calculate a motionvector, and carries out an inter prediction process on the coding blockby using both the motion vector and the inter prediction parametersoutputted from the coding controlling part 1 to generate an interprediction image P_(i) ^(n) (step ST6).

The motion-compensated prediction part 5 outputs the inter predictionimage P_(i) ^(n) to the subtracting part 6 and the adding part 9 aftergenerating the inter prediction image P_(i) ^(n), while outputting theinter prediction parameters to the variable length coding part 13 toenable the image decoding device shown in FIG. 3 to generate the sameinter prediction image P_(i) ^(n). The inter prediction parametersinclude the following pieces of information:

-   -   (1) Mode information in which the division of the coding block        B^(n) into partitions P_(i) ^(n) is described;    -   (2) The motion vector of the partition P_(i) ^(n);    -   (3) Reference image indication index information showing which        reference image is used for performing an inter prediction        process when the motion-compensated prediction frame memory 12        stores a plurality of local decoded images (reference images);    -   (4) Index information showing which motion vector predicted        value is selected and used when there are a plurality of motion        vector predicted value candidates;    -   (5) Index information showing which filter is selected and used        when there are a plurality of motion compensation interpolation        filters; and    -   (6) Selection information showing which pixel accuracy is used        when the motion vector of the partition P_(i) ^(n) can show a        plurality of degrees of pixel accuracy (half pixel, ¼ pixel, ⅛        pixel, etc.).

After the intra prediction part 4 or the motion-compensated predictionpart 5 generates a prediction image (an intra prediction image P_(i)^(n) or an inter prediction image P_(i) ^(n)), the subtracting part 6subtracts the prediction image (the intra prediction image P_(i) ^(n) orthe inter prediction image P_(i) ^(n)) generated by the intra predictionpart 4 or the motion-compensated prediction part 5 from the partitionP_(i) ^(n) of the coding block B^(n), which is generated through thedivision by the block dividing part 2, to generate a difference image,and outputs a prediction difference signal e_(i) ^(n) showing thedifference image to the transformation/quantization part 7 (step ST7).

When receiving the prediction difference signal e_(i) ^(n) showing thedifference image from the subtracting part 6, thetransformation/quantization part 7 carries out a transforming process(e.g., a DCT (discrete cosine transform) or an orthogonal transformationprocess, such as a KL transform, in which bases are designed for aspecific learning sequence in advance) on the difference image in unitsof a block having the transformation block size included in theprediction difference coding parameters outputted thereto from thecoding controlling part 1, and quantizes the transform coefficients ofthe difference image by using the quantization parameter included in theprediction difference coding parameters and outputs the transformcoefficients quantized thereby to the inverse quantization/inversetransformation part 8 and the variable length coding part 13 ascompressed data of the difference image (step ST8).

When receiving the compressed data of the difference image from thetransformation/quantization part 7, the inverse quantization/inversetransformation part 8 inverse-quantizes the compressed data of thedifference image by using the quantization parameter included in theprediction difference coding parameters outputted thereto from thecoding controlling part 1, performs an inverse transformation process(e.g., an inverse DCT (inverse discrete cosine transform) or an inversetransformation process such as an inverse KL transform) on thecompressed data inverse-quantized thereby in units of a block having thetransformation block size included in the prediction difference codingparameters, and outputs the compressed data on which the inversequantization/inverse transformation part has carried out the inversetransformation process as a local decoded prediction difference signalto the adding part 9 (step ST9).

When receiving the local decoded prediction difference signal from theinverse quantization/inverse transformation part 8, the adding part 9adds the local decoded prediction difference signal and the predictionsignal showing the prediction image (the intra prediction image P_(i)^(n) or the inter prediction image P_(i) ^(n)) generated by the intraprediction part 4 or the motion-compensated prediction part 5 togenerate a local decoded image which is a local decoded partition imageor a local decoded coding block image which is a group of local decodedpartition images (step ST10). After generating the local decoded image,the adding part 9 stores a local decoded image signal showing the localdecoded image in the memory 10 for intra prediction and also outputs thelocal decoded image signal to the loop filtering part 11.

When receiving the local decoded image signal from the adding part 9,the loop filtering part 11 carries out a filtering process on the localdecoded image signal to remove a distortion (block distortion) occurringat a block boundary (step ST11). Although the details of the processingcarried out by the loop filtering part 11 will be mentioned below, whenremoving a block distortion of the local decoded image, the loopfiltering part 11 sets the intensity of a filter for removing the blockdistortion for each of the signal components (the luminance signalcomponent and the color difference signal components) according to thecoding mode (an intra coding mode or an inter coding mode) selected bythe coding controlling part 1. The loop filtering part 11 can carry outthe filtering process on each coding block having the maximum size ofthe local decoded image signal outputted thereto from the adding part 9or each coding block. As an alternative, after the local decoded imagesignals corresponding to all the macroblocks of one screen areoutputted, the loop filtering part can carry out the filtering processon all the macroblocks of the one screen at a time.

The image coding device repeatedly carries out the processes of stepsST4 to ST10 until the image coding device completes the processing onall the coding blocks B^(n) into which the inputted image is dividedhierarchically, and, when completing the processing on all the codingblocks B^(n), shifts to a process of step ST13 (step ST12).

The variable length coding part 13 entropy-encodes the compressed dataoutputted thereto from the transformation/quantization part 7, thecoding mode (including the information showing the state of the divisioninto the coding blocks) and the prediction difference coding parameters,which are outputted thereto from the coding controlling part 1, and theintra prediction parameter outputted thereto from the intra predictionpart 4 or the inter prediction parameters (including the motion vector)outputted thereto from the motion-compensated prediction part 5. Thevariable length coding part 13 multiplexes coded data which are thecoded results of the entropy coding of the compressed data, the codingmode, the prediction difference coding parameters, and the intraprediction parameter or the inter prediction parameters to generate abitstream (step ST13).

Hereafter, the filtering process carried out by the loop filtering part11 will be explained concretely. The loop filtering part 11 is anonlinear smoothing filter for reducing a block noise which can occur ata boundary between partitions or transformation blocks each of which isa prediction unit mentioned above. FIG. 7 is an explanatory drawingshowing the positions of pixels in a coding block to which the filter isapplied. In FIG. 7 , a position where a vertical edge or a horizontaledge overlaps a boundary between partitions or transformation blocks issubjected to the filtering process. In FIG. 7 , each of the vertical andhorizontal edges is expressed by a K×K-pixel grid. The value of K can bedetermined to be a fixed value, or can be set according to the maximumsize of the coding block, the maximum size of each of the partitions ortransformation blocks, or the like.

FIG. 8 is a flow chart showing the process carried out by the loopfiltering part 11. The loop filtering part 11 carries out the filteringprocess on each of the coding blocks. The loop filtering part 11determines whether a vertical edge or a horizontal edge overlaps aboundary between partitions or transformation blocks first (step ST41).When there is a vertical edge or a horizontal edge overlapping aboundary between partitions or transformation blocks, the loop filteringpart 11 carries out determination of the intensity of the filter appliedto the part where the vertical edge or the horizontal edge overlaps theboundary (steps ST42 and ST43). A method of determining the filterintensity will be mentioned below.

After determining the intensity of the filter, the loop filtering part11 carries out the filtering process while changing the final intensityof the filter according to the result of the determination of the filterintensity and the amount of change in the values of pixels which are thetarget for the filtering process (steps ST44 and ST45). A method for usein the filtering process will be mentioned below. The loop filteringpart 11 repeatedly carries out the processes of steps ST41 to ST45 untilthe processes on all the coding blocks in the picture are completed(step ST46). Identification information showing whether the loopfiltering part carries out this loop filtering process on each of allthe coding blocks in each slice is multiplexed into its slice header,and the image coding device is constructed in such a way as to determinethe value of the identification information according to the conditionsand transmit the value to the image decoding device.

Next, the process of determining the filter intensity which is carriedout by the loop filtering part 11 will be explained. FIG. 9 is a flowchart showing a method of determining the filter intensity. The loopfiltering part 11 determines the filter intensity bS for all of pixelsadjacent to a vertical edge or a horizontal edge according to thefollowing conditions (step ST51). In the following explanation, eachpixel close to an edge is expressed by the following symbol p_(i) (i=0,1, 2, 3) or q_(j) (j=0, 1, 2, 3), and a relationship between theposition of an edge and the position of each pixel is defined as shownin FIG. 10 .

The coding blocks include blocks for luminance signal component andblocks for color difference signal components, and the loop filteringpart 11 determines the filter intensity according the followingconditions.

-   -   (1) Whether the coding mode of the coding block which is the        target for filtering process is an intra coding mode or an inter        coding mode.    -   (2) Whether the signal component which is the target for        filtering process is a luminance signal component or a color        difference signal component?    -   (3) Whether or not a non-zero transform coefficient is included        in the transformation block including the pixel which is the        target for filtering process.    -   (4) The state of the motion parameter in the partition including        the pixel which is the target for filtering process.

The loop filtering part 11 determines the filter intensity according tothe following procedure.

(Step 1)

When an edge is located at a boundary between coding blocks, and thecoding mode of the coding block including p₀ or the coding blockincluding q₀ is an “intra coding mode,” the loop filtering partdetermines the filter intensity for the target signal component forfiltering process that is a luminance signal component as bS=2, anddetermines the filter intensity for the target signal component forfiltering process that is a color difference signal component as bS=4.

(Step 2)

When the condition shown in step 1 is not satisfied, and the coding modeof the coding block including p₀ or the coding block including q₀ is an“intra coding mode,” the loop filtering part determines the filterintensity for the target signal component for filtering process that isa luminance signal component as bS=1, and determines the filterintensity for the target signal component for filtering process that isa color difference signal component as bS=3.

(Step 3)

When the conditions shown in steps 1 and 2 are not satisfied, and p₀ orq₀ belongs to a transformation block having a non-zero orthogonaltransformation coefficient, the loop filtering part determines thefilter intensity for the target signal component for filtering processthat is a luminance signal component as bS=2, and determines the filterintensity for the target signal component for filtering process that isa color difference signal component as bS=2.

(Step 4)

When the conditions shown in steps 1 to 3 are not satisfied, and one ofthe following conditions is satisfied, the loop filtering partdetermines the filter intensity for the target signal component forfiltering process that is a luminance signal component as bS=1, anddetermines the filter intensity for the target signal component forfiltering process that is a color difference signal component as bS=1.

[Conditions]

-   -   The partition including p₀ and the partition including q₀ have        different reference pictures or different numbers of motion        vectors.    -   Each of the partition including p₀ and the partition including        q₀ uses a single motion vector, and the horizontal or vertical        component of each of the motion vectors has a difference        absolute value of 4 or more of ¼ pixel accuracy.    -   Each of the partition including p₀ and the partition including        q₀ uses two motion vectors, and, in at least one pair of motion        vectors (pair of a motion vector in p₀ and a motion vector in        q₀) each of which refers to the same reference picture, the        horizontal or vertical component of each of the motion vectors        has a difference absolute value of 4 or more of ¼ pixel        accuracy.        (Step 5)

When the conditions shown in steps 1 to 4 are not satisfied (also anedge other than those each located at a boundary between partitions ortransformation blocks does not satisfy these conditions), the loopfiltering part determines the filter intensity for the target signalcomponent for filtering process that is a luminance signal component asbS=0, and determines the filter intensity for the target signalcomponent for filtering process that is a color difference signalcomponent as bS=0.

When the coding mode of the coding block which is the target forprocessing is an intra coding mode, there is a high probability thatboth the luminance signal and the color difference signals have largeprediction residual electric power, and the distribution of thequantized transform coefficients differs greatly for each of the signalcomponents as compared with the time that an inter frame predictionusing an inter prediction is carried out. Because the degree of blockdistortion is influenced by how many effective transform coefficientsare subjectively lost due to the quantization, it is desirable to beable to adjust the value of the filter intensity which is a measure formeasuring the degree of block distortion for each of the luminance andcolor difference components particularly in the intra coding. While thefilter intensity for each of the color difference signal components isalways set to be the same value as that for the luminance signalcomponent in the conventional loop filter 106 (refer to FIG. 16 ), afilter intensity is set for each of the signal components (the luminancesignal component and the color difference signal components) accordingto the conditions in this Embodiment 1, and therefore a filter intensitywhich contributes to an improvement in the image quality as comparedwith the conventional loop filter is acquired for each of the signalcomponents.

After carrying out the determination of the filter intensity, the loopfiltering part 11 carries out the filtering process in the order ofvertical edges and horizontal edges on the basis of the result of thedetermination of the filter intensity. First, the loop filtering part 11calculates a maximum of the filter intensity bS for every K linespartially including a vertical edge, and sets the maximum as bSVer. Theloop filtering part 11 carries out the filtering process on pixelsadjacent to the edge in the K lines on the basis of the maximum bSVer.FIG. 11 is an explanatory drawing showing a unit in units of which thefiltering process is carried out on a vertical edge. The K×K pixelspartially including the vertical edge therein have the same bSVer. Thefinal filter intensity bS applied to each of the pixels is determined byboth the maximum bSVer and the amount of change in the pixel value ateach pixel position. FIG. 12 is an explanatory drawing showing a unit inunits of which the filtering process is carried out on a horizontaledge. The filtering process is the same as that on a vertical edge withthe exception that the direction of the process is a horizontal one.

Hereafter, the filtering process on the target pixel for processing willbe explained. The loop filtering part 11 carries out the filteringprocess in a direction of an vertical edge of the luminance signalcomponent according to the following procedure. Although the loopfiltering part 11 also carries out the filtering process in a directionof a horizontal edge of the luminance signal component according to thesame procedure, the loop filtering part uses the maximum bSHor of thefilter intensity bS for every K lines partially including the horizontaledge instead of the maximum bSVer.

[a] In the case of bSVer=0

The loop filtering part does not carry out the filtering process.

[b] In the case of bSVer≤2

-   -   (1) The loop filtering part determines parameters β and Tc in        the case of Q=(the qP value of the luminance) in FIG. 13 . FIG.        13 is an explanatory drawing showing a correspondence between Q        (the qP value of the luminance), and the parameters β and Tc.    -   (2) Determine        d=|p₂−2*p₁+p₀|+|q₂−2*q₁+q₀|+|p₂−2*p₁+p₀|+|q₂−2*q₁+q₀|, and        perform a filter calculation according to the following        conditions. When d is smaller than β>>2, |p₃−p₀|+|q₆-q₃| is        smaller than β>>2, and |p₀−q₀| is smaller than ((5*t_(c)+1)>>1).        p ₀′=Clip1_(Y)((p ₂+2*p ₁+2*p ₀+2*q ₀ +q ₁+4)>>3)        p ₁′=Clip1_(Y)((p ₂ +p ₁ +p ₀ +q ₀+2)>>2)        p ₂′=Clip1_(Y)((2*p ₃+3*p ₂ +p ₁ +p ₀ +q ₀+4)>>3)        q ₀′=Clip1_(Y)((p ₁+2*p ₀+2*q ₀+2*q ₁ +q ₂+4)>>3)        q ₁′=Clip1_(Y)((p ₀ +q ₀ +q ₁ +q ₂+2)>>2)        q ₂′=Clip1_(Y)((p ₀ +q ₀ +q ₁+3*q ₂+2*q ₃+4)>>3)        Otherwise,        Δ=Clip3(−t _(C) ,t _(C),(13*(q ₀−₀)+4*(q ₁ −p ₁)−5*(q ₂ −p        ₀)+16)>>5)        p ₀′=Clip1_(Y)(p ₀+Δ)        q ₀′=Clip1_(Y)(q ₀−Δ)        p ₁′=Clip1_(Y)(p ₁+Δ/2)        q ₁′=Clip1_(Y)(q ₁−Δ/2)    -   (3) Convert p₀′ to p₂′ and q₀′ to q₂′, which are determined as        above, to values of pixels p₀ to p₂ and q₀ to q₂, and output        converted image as decoded image to motion-compensated        prediction frame memory 12 at next stage.        [c] In the case of bSVer>2

The loop filtering part determines the parameter β in the case of Q=(theqP value of the luminance) and the parameter Tc in the case of Q=(the qPvalue of the luminance+4) in FIG. 13 . The loop filtering part carriesout the subsequent processes in the same way that the loop filteringpart does in the case of bSVer≤2.

The loop filtering part 11 carries out the filtering process in adirection of a vertical edge of each of the color difference signalcomponents according to the following procedure. Although the loopfiltering part 11 also carries out the filtering process in a directionof a horizontal edge of each of the color difference signal componentsaccording to the same procedure, the loop filtering part uses themaximum bSHor instead of the maximum bSVer. bSVer and bSVer are thevalues calculated for the luminance at the same position.

[a] In the case of bSVer>2

-   -   (1) Perform the following filter calculation.        Δ=Clip3(−t _(C) ,t _(C),((((q ₀ −p ₀)<<2)+p ₁ −q ₁+4)>>3))        p ₀′=Clip1_(C)(p ₀+Δ)        q ₀′=Clip1_(C)(q ₀−Δ)    -   (2) Convert p₀′ and q₀′, which are determined as above, to        values of pixels p₀ and q₀, and output converted image as        decoded image to motion-compensated prediction frame memory 12        at next stage.        [b] In the case of bSVer≤2

The loop filtering part does not carry out the filtering process.

While the loop filtering part 11 of the image coding device and the loopfiltering part 28 of the image decoding device carry out a commonprocess, the loop filtering part 11 of the image coding device can beconstructed in such a way as to have a parameter for control for thedetermination of the filter intensity. For example, the image codingdevice can be constructed in such a way as to multiplex identificationinformation showing whether or not to carry out signaling of the valueof the filter intensity bS on the slice level, thereby making itpossible to change the setting of the filter intensity bS on a per slicebasis. At this time, the image coding device enables only the setting ofthe filter intensity for the luminance signal component to be changedwhile fixing the setting of the filter intensity for each of the colordifference signal components. In contrast with this, the image codingdevice enables only the setting of the filter intensity for each of thecolor difference signal components to be changed while fixing thesetting of the filter intensity for the luminance signal component. Theimage coding device can carry out the signaling of the setting of thefilter intensity by simply transmitting the setting itself. The imagecoding device can alternatively carry out the signaling of the settingof the filter intensity by transmitting an offset from the default valueof the filter intensity bS. The filter intensity for each of the colordifference signal components can be expressed as an offset from thefilter intensity for the luminance signal component.

Further, the image coding device can also be constructed in such a wayas to carry out signaling of the setting of the filter intensity bS bysimply transmitting the setting itself or an offset from the defaultvalue particularly in the case in which the coding mode is an intracoding mode. For example, the loop filtering part 11 can be constructedin such a way as to determine the filter intensity according to thefollowing procedure.

When the intensity value of the filter used for the luminance componentof a coding block coded in an intra coding mode is expressed as bSL andthe intensity value of the filter used for each color differencecomponent of the coding block is expressed as bSC, both bSL and bSC aremultiplexed into the bitstream as syntax information,

such as a header on the picture level or a slice header, so that bSL andbSC can be shared between the coding device and the decoding device.

(Step 1)

At this time, when an edge is located at a boundary between codingblocks, and the coding mode of the coding block including p₀ or thecoding block including q₀ is an “intra coding mode,” the loop filteringpart determines the filter intensity for the target signal component forfiltering process that is a luminance signal component as bS=max(4-bSL,0), and determines the filter intensity for the target signal componentfor filtering process that is a color difference signal component asbS=max(4-bSC, 0). max(A, B) is a function of outputting the larger oneof A and B.

(Step 2)

When the condition shown in step 1 is not satisfied, and the coding modeof the coding block including p₀ or the coding block including q₀ is an“intra coding mode,” the loop filtering part determines the filterintensity for the target signal component for filtering process that isa luminance signal component as bS=max(3-bSL, 0), and determines thefilter intensity for the target signal component for filtering processthat is a color difference signal component as bS=max(3-bSC, 0).

When the coding mode is an intra coding mode, there are a case in whichthe intra coding is carried out unavoidably without a motion predictionfunctioning effectively in the compression process, and a case in whichthe intra coding is carried out periodically and intentionally from theviewpoint of error resistance and random access. In the case in whichthe intra coding is carried out unavoidably, an additional distortion issuperimposed according to the difficulty of the coding. In contrast withthis, in the case in which the intra coding is carried out periodicallyand intentionally, a difference appears in the occurrence of a blockdistortion because the intra coding is directly used regardless of thedifficulty of the coding. A conventional loop filter has no means ofdiscriminating between these cases to control the filter intensity.Because periodic intra-frame insertion is carried out on a per slicebasis or on a per picture basis, controlling the filter intensity on aper slice basis or on a per picture basis according to the use to whichthe image coding device is put makes it possible to prevent theoccurrence of a block distortion more effectively. As an alternative,the image coding device can be constructed in such a way as to carry outsignaling of the setting of the filter intensity bS at the time that thecoding mode is an inter coding mode.

FIG. 14 is an explanatory drawing showing the bitstream generated by thevariable length coding part 13. In the example shown in FIG. 14 , astate in which slice coded data consist of a slice header and a numberof the coded data of largest coding blocks, which follow the sliceheader, in the slice is shown. The coded data of each largest codingblock includes coding mode information. Although not illustrated, thecoded data of each largest coding block includes prediction parameters,such as the motion vector of each partition, prediction differencecoding parameters, such as a transformation block size, and predictiondifference coded data (quantized transform coefficients). The sliceheader includes a loop filter ON/OFF flag that is identificationinformation showing whether or not to carry out the loop filteringprocess on each of all the coding blocks in the slice, a filterintensity information multiplexing flag indicating whether or not tocarry out signaling of the setting of the filter intensity bS, andfilter intensity information that is multiplexed into the bitstream whenthe filter intensity information multiplexing flag is “1.” The filterintensity information multiplexing flag and the filter intensityinformation can be configured in such a way as to be multiplexed into anheader information region that is defined on a per picture, sequence, orGOP (Group Of Pictures) basis.

Next, the processing carried out by the image decoding device shown inFIG. 3 will be explained. When receiving the bitstream outputted fromthe image coding device shown in FIG. 1 , the variable length decodingpart 21 carries out a variable length decoding process on the bitstream(step ST21 of FIG. 4 ), and decodes the information for defining thepicture size (the number of horizontal pixels and the number of verticallines) on a per sequence basis, each sequence consisting of one or moreframes of pictures, or on a per picture basis.

The variable length decoding part 21 determines a maximum size of eachof coding blocks which is a unit to be processed at a time when amotion-compensated prediction process (inter-frame prediction process)or an intra prediction process (intra-frame prediction process) iscarried out according to the same procedure as that which the codingcontrolling part 1 shown in FIG. 1 uses, and also determines an upperlimit on the number of hierarchical layers in a hierarchy in which eachof the coding blocks having the maximum size is hierarchically dividedinto blocks (step ST22). For example, when the maximum size of each ofcoding blocks is determined according to the resolution of the inputtedimage in the image coding device, the variable length decoding partdetermines the maximum size of each of the coding blocks on the basis ofthe frame size which the variable length decoding part has decodedpreviously. When information showing both the maximum size of each ofthe coding blocks and the upper limit on the number of hierarchicallayers is multiplexed into the bitstream, the variable length decodingpart refers to the information which is acquired by decoding thebitstream. When the bitstream has a structure shown in FIG. 14 , thevariable length decoding part 21 decodes the loop filter ON/OFF flagfrom the slice header in advance of decoding each largest coding block.

Because the information showing the state of the division of each of thecoding blocks B⁰ having the maximum size is included in the coding modem(B⁰) of each coding block B⁰ having the maximum size which ismultiplexed into the bitstream, the variable length decoding part 21specifies each of the coding blocks B^(n) into which the image isdivided hierarchically by decoding the bitstream to acquire the codingmode m(B⁰) of the coding block B⁰ having the maximum size which ismultiplexed into the bitstream (step ST23). After specifying each of thecoding blocks B^(n), the variable length decoding part 21 decodes thebitstream to acquire the coding mode m(B^(n)) of the coding block B^(n)to specify each partition P_(i) ^(n) belonging to the coding block B^(n)on the basis of the information about the partition P_(i) ^(n) belongingto the coding mode m(B^(n)). After specifying each partition P_(i) ^(n)belonging to the coding block B^(n), the variable length decoding part21 decodes the coded data to acquire the compressed data, the codingmode, the prediction difference coding parameters, and the intraprediction parameter or the inter prediction parameters (including themotion vector) for each partition P_(i) ^(n) (step ST24).

When the coding mode m(B^(n)) of the partition P_(i) ^(n) belonging tothe coding block B^(n), which is outputted from the variable lengthdecoding part 21, is an intra coding mode (step ST25), the selectionswitch 22 outputs the intra prediction parameter outputted thereto fromthe variable length decoding part 21 to the intra prediction part 23. Incontrast, when the coding mode m(B^(n)) of the partition P_(i) ^(n) isan inter coding mode (step ST25), the selection switch outputs the interprediction parameters outputted thereto from the variable lengthdecoding part 21 to the motion-compensated prediction part 24.

When receiving the intra prediction parameter from the selection switch22, the intra prediction part 23 carries out an intra prediction processon the partition P_(i) ^(n) of the coding block B^(n) by using the intraprediction parameter to generate an intra prediction image P_(i) ^(n)while referring to the decoded image (reference image) of analready-decoded block stored in the memory 27 for intra prediction (stepST26).

When receiving the inter prediction parameters outputted from theselection switch 22, the motion compensation part 24 carries out aninter prediction process on the coding block by using the motion vectorincluded in the inter prediction parameters and the decoded image(reference image) of an already-decoded block stored in themotion-compensated prediction frame memory 29 to generate an intraprediction image P_(i) ^(n) (step ST27).

The inverse quantization/inverse transformation part 25inverse-quantizes the compressed data outputted thereto from thevariable length decoding part 21 by using the quantization parameterincluded in the prediction difference coding parameters outputtedthereto from the variable length decoding part 21, and performs aninverse transformation process (e.g., an inverse DCT (inverse discretecosine transform) or an inverse transformation process such as aninverse KL transform) on the compressed data inverse-quantized therebyin units of a block having the transformation block size included in theprediction difference coding parameters, and outputs the compressed dataon which the inverse quantization/inverse transformation part hascarried out the inverse transformation process to the adding part 26 asa decoded prediction difference signal (signal showing a pre-compresseddifference image) (step ST28).

When receiving the decoded prediction difference signal from the inversequantization/inverse transformation part 25, the adding part 26generates a decoded image by adding the decoded prediction differencesignal and the prediction signal showing the prediction image generatedby the intra prediction part 23 or the motion-compensated predictionpart 24 and stores the decoded image signal showing the decoded image inthe memory 27 for intra prediction, and also outputs the decoded imagesignal to the loop filtering part 28 (step ST29).

The image decoding device repeatedly carries out the processes of stepsST23 to ST29 until the image decoding device completes the processing onall the coding blocks B^(n) into which the image is dividedhierarchically (step ST30). When receiving the decoded image signal fromthe adding part 26, the loop filtering part 28 carries out a filteringprocess on the decoded image signal to remove a distortion (blockdistortion) occurring at a block boundary, and stores the decoded imageshown by the decoded image signal from which the distortion is removedin the motion-compensated prediction frame memory 29. The filteringprocess carried out by the loop filtering part 28 is the same as thatcarried out by the loop filtering part 11 shown in FIG. 1 , and, whenremoving a block distortion occurring in the decoded image, the loopfiltering part 28 sets the intensity of a filter for removing a blockdistortion for each of the signal components (the luminance signalcomponent and the color difference signal components) according to thecoding mode information (an intra coding mode or an inter coding mode)outputted from the variable length decoding part 21. When the variablelength decoding part 21 decodes the filter intensity informationmultiplexing flag and the filter intensity information from the sliceheader, the loop filtering part carries out the filtering process withthe filter intensity bS shown by the filter intensity information.

As can be seen from the above description, the loop filtering part 28 ofthe image decoding device in accordance with this Embodiment 1 isconstructed in such a way as to, when removing a block distortionoccurring in the decoded image, set the intensity of a filter forremoving the block distortion for each of the signal components (theluminance signal component and the color difference signal components)according to the coding mode information (an intra coding mode or aninter coding mode) outputted from the variable length decoding part 21.Therefore, there is provided an advantage of being able to improve theaccuracy of removal of a block distortion, thereby improving the qualityof the decoded image.

Although the example in which the image coding device carries out aninter-frame motion-compensated prediction process (inter predictionprocess) and the image decoding device carries out an inter-framemotion-compensated prediction process (inter prediction process) isshown in the above explanation, each of the loop filtering parts 11 and28 can be constructed in such a way as to remove a block distortion evenwhen the image coding device carries out an intra-frame predictionprocess (intra prediction process) on each of all the frames and theimage decoding device carries out an intra-frame prediction process(intra prediction process) on each of all the frames. In a case in whichthe image coding device is constructed in such a way as to carry out acombination of an intra-frame prediction process (intra predictionprocess) and an inter-frame motion-compensated prediction process (interprediction process), and the image decoding device is constructed insuch a way as to carry out a combination of an intra-frame predictionprocess (intra prediction process) and an inter-frame motion-compensatedprediction process (inter prediction process), it is possible to controlthe loop filtering parts 11 and 28 in such a way that these loopfiltering parts do not operate when all the frames are set to be codedthrough an intra prediction process.

Although the example in which the size of the coding block B^(n) isL^(n)=M^(n) as shown in FIG. 5 is shown in this Embodiment 1, the sizeof the coding block B^(n) can be L^(n)≠M^(n). For example, there can beconsidered a case in which the size of the coding block B^(n) isL^(n)=kM^(n) as shown in FIG. 15 . In this case, (L^(n+1), M^(n+1))becomes equal to (L^(n), M^(n)) in the next division, and subsequentdivisions can be carried out in the same way as those shown in FIG. 5 orin such a way that (L^(n+1), M^(n+1)) becomes equal to (L^(n)/2,M^(n)/2).

For example, by setting M⁰=16 using this configuration, a largest codingblock that is formed of horizontally-coupled macroblocks each of whichconsists of 16×16 pixels, like those defined in MPEG-2 (ISO/IEC 13818-2)or MPEG-4 AVC/H.264 (ISO/IEC 14496-10), can be defined, and there isprovided an advantage of being able to construct an image coding devicethat maintains compatibility with such an existing method. It isneedless to say that division can be carried out on even a largestcoding block that is formed of vertically-coupled macroblocks, such asmacroblocks in the case of not L^(n)=kM^(n) but kL^(n)=M^(n), under thesame idea.

While the invention has been described in its preferred embodiment, itis to be understood that various changes can be made in an arbitrarycomponent in accordance with the embodiment, and an arbitrary componentin accordance with the embodiment can be omitted within the scope of theinvention.

INDUSTRIAL APPLICABILITY

Because each of the image coding device and the image decoding device inaccordance with the present invention has a function of setting theintensity of a filter for each signal component according to the codingmode, thereby being able to improve the accuracy of removal of a blockdistortion and hence improve the quality of the coded image, and each ofthe image coding method and the image decoding method in accordance withthe present invention has a step of setting the intensity of a filterfor each signal component according to the coding mode, thereby beingable to improve the accuracy of removal of a block distortion and henceimprove the quality of the coded image, the image coding device, theimage decoding device, the image coding method, and the image decodingmethod can be applied to an international standard video coding method,such as MPEG or ITU-T H.26x.

EXPANATIONS OF REFERENCE NUMERALS

1 coding controlling part (coding mode determining unit), 2 blockdividing part (block division unit), 3 select switch (prediction imagegenerating unit), 4 intra prediction part (prediction image generatingunit), 5 motion-compensated prediction part (prediction image generatingunit), 6 subtracting part (difference image generating unit), 7transformation/quantization part (image compression unit), 8 inversequantization/inverse transformation part (local decoded image), 9 addingpart (local decoded image), 10 memory for intra prediction, 11 loopfiltering part (distortion removing unit), 12 motion-compensatedprediction frame memory, 13 variable length coding part (coding unit),21 variable length decoding part (decoding unit), 22 select switch(prediction image generating unit), 23 intra prediction part (predictionimage generating unit), 24 motion compensation part (prediction imagegenerating unit), 25 inverse quantization/inverse transformation part(difference image generating unit), 26 adding part (decoded imagegenerating unit), 27 memory for intra prediction, 28 loop filtering part(distortion removing unit), 12 motion-compensated prediction framememory, 101 block dividing part, 102 predicting part, 103 compressingpart, 104 local decoding part, 105 adder, 106 loop filter, 107 memory,108 variable length coding part.

The invention claimed is:
 1. An image decoding device that carries out adecoding process on a coded data obtained by performing a block basedcoding process on an image partitioned into one or more coding blocks,the image decoding device comprising: a coded data decoder to decode thecoded data to acquire compressed data of a difference image associatedwith each one of the coding blocks and an offset, a distortion removerto carry out a filtering process on a decoded image obtained by adding aprediction image to the difference image, and to remove a blockdistortion at a boundary between adjacent transform blocks of thedecoded image, the transform block being for transforming the differenceimage; and a memory storing decoded pixel values for outputting thedecoded image on which the filtering process has been carried out,wherein the distortion remover derives a value of a parameter specifyingfiltering strength as a default value for the offset according to a setof conditions including whether a coding mode corresponding to at leastone of the adjacent transform blocks to which the filtering process isto be applied is intra prediction mode, wherein the offset is added tothe value of the parameter in a process of selecting from differentfiltering processes.
 2. An image decoding method that carries out adecoding process on a coded data obtained by performing a block basedcoding process on an image partitioned into one or more coding blocks,the image decoding method comprising: decoding the coded data to acquirecompressed data of a difference image associated with each one of thecoding blocks and an offset, carrying out a filtering process on adecoded image obtained by adding a prediction image to the differenceimage, and removing a block distortion at a boundary between adjacenttransform blocks of the decoded image, the transform block being fortransforming the difference image; and storing decoded pixel values foroutputting the decoded image on which the filtering process has beencarried out, wherein the filtering process derives a value of aparameter specifying filtering strength as a default value for theoffset according to a set of conditions including whether a coding modecorresponding to at least one of the adjacent transform blocks to whichthe filtering process is to be applied is intra prediction mode, whereinthe offset is added to the value of the parameter in a process ofselecting from different filtering processes.